This application claims the priority of Korean Patent Application No. 2004-71222, filed on Sep. 7, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an optical amplifier, and more particularly, to a hybrid optical amplifier, a structure of which is more simplified, production of which is easy, and which is cost-effective, using a gain-clamped semiconductor optical amplifier enabling Raman amplification.
2. Description of the Related Art
The semiconductor optical amplifiers (SOAs) are optical amplifiers whose gain media are semiconductor. Even though the SOAs have various advantages such as compact size, easy wavelength-band selection, and wide wavelength bandwidth, fiber optical amplifiers are mainly used in the high-speed optical communication systems requiring the transmission rates of 2.5 Gbit/s or beyond since the signal crosstalk in the SOAs degrades transmission performance.
The signal crosstalk is understood in terms of gain-recovery time (GRT). For example, since the erbium-doped fiber amplifiers (EDFAs) have very slow gain dynamics compared with one-bit time slot (400 ps in 2.5 Gbit/s), the gain characteristics of the EDFAs are not changed during the fast signal modulation. The GRT of a typical EDFA is of the order of 10 ms. On the other hand, the SOAs have fast gain dynamics comparable to one-bit time slot. Since a typical SOA has the GRT of hundreds of ps (1 ps=10−12 s), it responds to the fast signal modulation and its gain characteristic is changed according to the modulated signal patterns. In other words the gain characteristic of the SOA is changed according to the intensity of current and/or previous signal, which is called time-division multiplexed (TDM) crosstalk. Additionally the changes in the neighboring channels' intensity cause the change of the gain characteristic when the wavelength-division multiplexed (WDM) signals are input to the SOA, which is called WDM crosstalk.
To use SOAs for the high-speed optical communication, the crosstalk described above must be suppressed. There are several methods that have been reported to suppress the crosstalk, and a few of them are introduced below.
First, there is a holding-light injection method where an extra injection laser is injected to an SOA. The use of the injection light makes the SOA respond quicker so that the resultant GRT can become a tenth of the usual GRT. Due to this fast gain-recovery compared with one-bit time slot the WDM/TDM crosstalk are effectively suppressed. However, this method requires a separate external laser, which raises cost and increases total size.
Second, there is a method of modifying the transmitters and receivers instead of improving the SOAs. For example, in a reference paper (IEEE Photonics Technology Letters Vol. 12, No. 10, A. K. Srivastava, et al., “A polarization multiplexing technique to mitigate WDM crosstalk in SOAs”), two channels of different wavelengths were split with 1:1 ratio, each branch was intensity modulated out of phase (180°), and finally they were multiplexed using polarization-beam combiners. Since the signal modulated using this method had constant signal intensity, negligible crosstalk due to the intensity variation occurred after being transmitted through the usual SOAs. Before the signals arrived at the receivers, they were demultiplexed by polarization-beam demultiplexer. A similar method was introduced in another reference paper (IEEE Photonics Technology Letters Vol. 12, No. 10, Hyang K. Kim, et al., “Reduction of cross-gain modulation in the semiconductor optical amplifier by using wavelength modulated signal”). In this method, the crosstalk was suppressed by keeping the intensity of the input signals constant by using the auxiliary signals modulated out of phase whose wavelengths were 0.3 nm apart from the main signals. However, in these methods the widely used conventional transceivers should be changed, which requires higher system cost.
Third, there is a method of inducing laser oscillation so that it is possible to perform gain clamping in an SOA itself. According to U.S. Pat. No. 5,184,247 titled “Optically Stabilized Feedback Amplifier,” a method of inducing laser oscillation using gratings for wavelength selection inside an SOA is disclosed. Also, according to U.S. Pat. No. 5,463,759 titled “Crosstalk Free, Low-noise Optical Amplifier,” a method of stabilizing amplified signal by inducing laser oscillation in a direction perpendicular to signal propagation in an SOA is disclosed. These optical amplifiers are called gain-clamped semiconductor optical amplifiers (GCSOAs). The GCSOAs have advantages of simple structure and low cost since the external lasers of the first method are unnecessary, and the transceivers do not need to be modified.
Two main features of the present invention are gain-clamping introduced above and Raman amplification. The Raman amplification is based on the process of stimulated Raman scattering (SRS) that transfers part of pump energy to signal energy. The SRS in a spool of optical fiber is generated by the interaction of light with the molecular vibration of doped material. In the process, the frequency shift from the pump of short wavelength to the signal of long wavelength can be as large as 40 THz (300 nm). In particular, when the frequency shift is 13.2 THz (105 nm at ˜1.55 μm, 73 nm at ˜1.3 μm), the Raman amplification has gain peak. This plays an important role in the Raman amplification. In other words, the optical amplification is possible in any wavelength band by selecting appropriate pumping wavelength. However, since the SRS effect is rather weak in the usual optical fiber, long length of the optical fiber and high pump power are necessary to get a Raman optical amplifier (ROA) with reasonably high gain.
The ROAs and GCSOAs described above may be complementary in terms of the gain and noise characteristics. It is known that the GCSOAs have lower optical power compared with the EDFAs. In the optical transmission systems using such low power GCSOAs, an allowable WDM channel count is few and a span between the GCSOAs is short. To overcome these limitations, the combined amplifiers of the ROAs and GCSOAs may replace the pure GCSOAs. For example, to get 20-dB gain per span, a 10-dB GCSOA is enough if a 10-dB ROA is used simultaneously. Moreover, if the 10-dB ROA is inserted before the 10-dB GCSOA, the effective noise figure is better than that of a single 20-dB SOA
FIG. 1 shows a schematic diagram of a conventional hybrid optical amplifier (HOA) where an ROA and an SOA are combined.
FIG. 1 refers to Korea Patent No. 0395430 entitled “Apparatus for coupling between raman optical fiber amplifier and semiconductor optical amplifier” wherein an HOA obtained by assembling a pump laser chip, an SOA chip, and a wavelength-selective coupler (WSC) into a package to effectively combine the ROA and the SOA is disclosed.
Referring to FIG. 1, the HOA 120 includes the optical fibers for transmission line 100 and 110, a laser diode chip 121, a dichroic mirror 124 passing incident signal beam passed through a first lens 122 and reflecting a laser beam input from the ROA laser diode chip 121 in the opposite direction to the incident signal beam, an isolator 125 isolating a Raman optical amplification part from a semiconductor amplification part, an SOA chip 127 amplifying the optical signal passed through the isolator 125, a second lens 123 emitting the laser beam generated by the ROA laser diode chip 121 to the dichroic mirror 124, a third lens 126 condensing the optical signal transmitted from the isolator 125 and emitting the condensed optical signal to the SOA chip 127, and a fourth lens 128 condensing the signal beam output from the SOA chip 127 to input to the optical fiber 110 in one package. Here, the Raman optical amplification laser provided to the dichroic mirror 124 can be provided by the ROA laser diode chip 121 included in the HOA 120 or provided outside.
The HOA described above includes the pump laser chip for Raman pumping, the WSC and the GCSOA chip. If all the components are to be packaged into one, manufacturing becomes difficult and the corresponding cost may rise. On the contrary, the present invention does not need a pump laser chip and a WSC for an HOA.